Enrichment of silicon for a better kilogram

A metrological challenge is currently underway to replace the present definition of the kilogram by the mass of a certain number of silicon atoms. A prerequisite for this is that the Avogadro constant, N-A, which defines the number of atoms in a mole, is determined with a relative uncertainty of better than 2 x 10(-8). Silicon crystals are used for this determination, the difficulty arising thereby is the measurement of the average molar mass of natural Si. Consequently, a worldwide collaboration has been launched to produce similar to 5kg of Si-28 single crystal with an enrichment factor greater than 0.99985 and of sufficient chemical purity so that it can be used to determine NA with the targeted relative measurement uncertainty mentioned above. In the following, the development and first successful tests of all technological steps are reported, and the new equipment for the production of high-purity Si-28 with an enrichment of not less than 0.9999 is described. All steps are defined by a Technical Road Map (TRM28) mandatory for all partners, and all key results are measured by calibrated and certified means, e.g. the C content of the final material is less than 10(15) atoms/cm(3) and the specific resistance is in the range from 400 to 1000 Omega cm. New applications based on this highly enriched and purified Si-28, and on Si-29 and Si-30 monocrystals produced in parallel, are reported briefly in the fields of solid state spectroscopy, spintronics, quantum computing, cooling of highly loaded SYS optics, superlattice structure (SLS), terahertz laser. 0 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim